Operational implementation of RTTOV-11 in the IFS

An interpolation algorithm is incorporated into RTTOV to map the input profiles to the RTTOV model coordinate and the Jacobians from the RTTOV model coordinate to the forecast model coordinate. The default in the ECMWF's IFS is the Rochon interpolator that was implemented in RTTOV-9 and remained unchanged in the RTTOV-10 upgrade. Experimental results indicate that the default interpolator commonly used operationally may show unphysical oscillations in Jacobians that may cause unphysical analysis increments. The latest development of the radiative transfer code RTTOV-11 has been enhanced to provide improved profile interpolation from user levels to RTTOV levels and back again. This memorandum summarises the evaluation of RTTOV-11 for use in the operational assimilation system at ECMWF. Two of the new interpolation options (modes 3 and 5) are evaluated through comparisons of radiative transfer simulations with RTTOV-11 using default Rochon interpolator, and through an analysis of departure characteristics against observations. The impact on forecasts is also investigated through a series of assimilation experiments. Results show that enabling interpolation mode 5 ensures smooth Jacobians profiles required by the data assimilation calculations when the input profile levels are more densely-spaced than the coefficient levels. The performance in the IFS as measured by forecast skill, fit to observations and model run time supports the operational use of the RTTOV-11 and interpolation mode 5 with the ECMWF IFS cycle 41r1 in May 2015.